Fix json output for none type

For non-json output we print an empty string
("") for a none type, however for json output
we need to "quote" this empty string so we
don't get invalid json e.g.

```
bpftrace -e 'kprobe:vfs_write {print(*(struct file *)arg0); exit()}' -f json
```

Before snippet:
```
{"type": "value", "data": { "f_u": , "f_path": { "mnt": 18446612686587658272, ...
```

After snippet:
```
{"type": "value", "data": { "f_u": "", "f_path": { "mnt": 18446612686587658272, ...
```

This is ultimately caused by bpftrace treating
anon union and structs as none types.

Issue: bpftrace#3543

Author: Jordan Rome

CHANGELOG.md

@@ -20,6 +20,8 @@ and this project adheres to
 #### Deprecated
 #### Removed
 #### Fixed
+- Fix json output for none type
+  - [#3692](https://github.com/bpftrace/bpftrace/pull/3692)
 #### Security
 #### Docs
 #### Tools

src/output.cpp

@@ -24,6 +24,7 @@ bool is_quoted_type(const SizedType &ty)
     case Type::inet:
     case Type::kstack_t:
     case Type::ksym_t:
+    case Type::none:
     case Type::strerror_t:
     case Type::string:
     case Type::timestamp:
@@ -40,7 +41,6 @@ bool is_quoted_type(const SizedType &ty)
     case Type::mac_address:
     case Type::max_t:
     case Type::min_t:
-    case Type::none:
     case Type::pointer:
     case Type::reference:
     case Type::record:

tests/runtime/json-output

@@ -178,3 +178,10 @@ EXPECT {"type": "value", "data": 333}
 NAME enum_symbolized_printf
 RUN {{BPFTRACE}} -q -f json -e 'enum Foo { ONE = 1, TWO = 2, OTHER = 99999 }; BEGIN { printf("%d %s %d %s %d %s\n", ONE, ONE, TWO, TWO, OTHER, OTHER); exit() }'
 EXPECT {"type": "printf", "data": "1 ONE 2 TWO 99999 OTHER\n"}
+
+# Note: if we fix the underlying issue, which is that we don't support unions
+# or anon structs, then we will need to change/delete this test
+NAME none type
+RUN {{BPFTRACE}} -q -f json -e 'union N { int i; float f; }; struct Foo { int m; union N n; }; uprobe:./testprogs/struct_with_union:func { print(*((struct Foo *) arg0)); exit(); }'
+EXPECT {"type": "value", "data": { "m": 2, "n": { "i": 5, "f": "" } }}
+AFTER ./testprogs/struct_with_union

tests/testprogs/struct_with_union.c

@@ -0,0 +1,23 @@
+union N {
+  int i;
+  float f;
+};
+
+struct Foo {
+  int m;
+  union N n;
+};
+
+int func(struct Foo *foo)
+{
+  return foo->m;
+}
+
+int main()
+{
+  struct Foo foo;
+  foo.m = 2;
+  foo.n.i = 5;
+  func(&foo);
+  return 0;
+}